1. Field of the Invention
The present invention is related to the field of photodetector and more particularly to a pixel cell with high light sensitivity and low delay time, wherein the photons are detected by a diode of the pixel cell.
2. Description of the Prior Art
Because the photons of incident light can be translated into the electron-hole pairs and the quantity of the induced current is proportional to the quantity of the absorbed photons. The semiconductor devices are broadly used to form the photodetector, especially are used to form the sensors of the photodetector.
Though there are many varieties of sensors, such as the high complementary metal-oxide semiconductor sensor (CMOS sensor) that within high integration and low energy dissipation, the structure of most of the conventional pixel cells can be divided into a circuit region and a diode region. Herein, the diode region is consist of a diode that is activated by the incident light, and the circuit region is consist of some devise that are linked to the diode and are used to process the signals induced by the induced current.
The conventional structure of the pixel cell is illustrated in FIG. 1 and explained in following discussions. Herein, the top-view figure of a CMOS sensor is shown.
As FIG. 1 shows, isolation 10 not only locates on the boundary of the pixel cell and encompasses it, but also briefly divides the pixel cell into a circuit region and a diode region. Herein, some devices such as gate 11 and conductive line 12 are located inside the circuit region and are used to process so-called induced current from the diode region. Beside, the pixel cell is formed on a substrate within a well, where the conductive type of the substrate is equal to the conductive type of the well. Further, doped regions 14 are formed in the substrate and are contiguous to the surface of substrate, and the conductive type of doped regions 14 is opposite to the conductive type of the well. Moreover, doped regions 14 are used to provide sources and drains of devices (gates 11 and conductive line 12) in the circuit region, and are used to provide the required diode in the diode region. Conductive structure 13 is used to connect one gate 11, which located inside the circuit region, and one conductive line 12, which located inside the diode region and contacted with doped region 14. As usual, in order to increase the protective area to enhance sensitivity of the pixel cell, doped regions 14 cover almost total surface of the substrate.
The conventional structure and mechanism of the pixel cell are further explained in following discussion with FIG. 2, where a qualitative cross-section illustration along the AAxe2x80x2 line is presented.
As FIG. 2 shows, when light is projected into the pixel cell, light is only projected in the diode region. Clearly, a diode is formed by doped region 14 and well 15, and then depletion region 16 is formed to absorb photons of incident light. Of course, in order to prevent depletion region 16 of a pixel cell interferes with other pixel cells, channel stopper 17 could be formed under isolation 10. Herein, isolation 10 can be the field oxide or the shallow junction. Moreover, FIG. 2 illustrates the case that three gates 11 and three conductive lines 12 are used, these gates 11 and conductive lines 12 are corresponding to reset terminal, raw select terminal and source follower terminal respectively.
Significantly, owing to that fact that doped regions 14 are usually produced at the same time and are used to form both diode and sources/drains, density and depth of doped regions 14 must balance requirements of both sources/drains and diode. Thus, the diode is not optimized to let the absorbing efficiency of depletion region 16 be maximized.
No matter how, even doped regions 14 and other regions are separately formed to maximize the absorbing efficiency of depletion region 16. Some defects of the previous pixel cell still are happened. First, because depletion region 16 is located under doped regions 14 and the penetrating depths of different wavelength lights are different, it is hard to form a pixel cell that is efficiently for different color lights simultaneously. For example, when the depth of doped regions 16 is larger than the penetrating depth of a specific light, the pixel cell is not efficient to detect the specific light. Second, because the narrow width of depletion region 16 will induce larger capacitance, resistance-capacitance (RC) delay time of the pixel cell is large and then the response of the pixel cell, such as reset time and readout time, would be prolonged. Third, the corner of isolation 10 is neighboring to the PN junction of the light induced diode, and then any damage of isolation 10 will induce leakage, i.e. dark current, especially when isolation 10 is a field oxide with high stress.
According to these discussions, it is crystal-clear that the conventional structure of pixel cell is not perfect and some defects are serious and unavoidable. Thus, it is desired to develop a new pix cell structure that efficiently overcomes these previous defects.
One primary object of the present invention is to propose a photodetector pixel cell with high absorbing efficiency.
Another object of the present invention is to propose a photodetector pixel cell with high light sensitevity and low RC delay time.
A further object of the present invention is to propose a photodetector pixel cell without negligible leakage current.
Moreover, a specific object is to propose a manufacturable photodetector pixel cell with the photodiode.
In order to accomplish these objects of the invention, a photodetector pixel cell with the photodiode is presented as an embodiment of the invention.
The presented pixel cell is enclosed by the isolation and comprises a diode region and a circuit region. Herein, the circuit region and the doped region also are briefly separated by the isolation. Moreover, the presented pixel cell is formed in and on a substrate, and the doped regions are located inside both the circuit region and the diode region. Further the conductive type of doped regions is opposite to the conductive type of the substrate.
Additional, the presented pixel cell at least comprises following characteristics: First, a well is only located in the circuit region, where the conductive type of the well is equal to the conductive type of the substrate. Second, a well is located inside the diode region and is contiguous to the isolation, where the conductive type of the well is equal to that of the doped region. Third, the doped regions are not contiguous to the second well. Fourth, the diode is provided by the doped region and the substrate.
According to previous characteristics, the scale of the PN-junction is increased and the width of the depletion region also is increased, and then the light sensitivity is increased and the RC delay time is decreased. Moreover, because the edge of the isolation is surrounded by the second well inside the diode region, the damage induced leakage current is properly prevented.